We present direct observations of period doubling in the flash to flash pulse heights in single-bubble sonoluminescence. States involved are stable, spherically symmetry broken. Observations are made using seven detectors distributed in the equatorial plane of the bubble. Contrary to earlier experiments by Holt et al. [Phys. Rev. Lett. 72, 1376 (1994)], where period doubling was observed in the time intervals between flashes but not in the pulse heights, we observe period doubling in pulse heights, but no corresponding period doubling is seen in the time intervals. In parameter space the period doubling is observed below the n=2 shape instability boundary line where extinction is shown to take place.
This paper presents a new advancement within the field of optical gas measurement with applications to the monitoring of gases inside the annulus of flexible risers used in the offshore industry. This advancement is based on the novel Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) technology. Specifically, we report on the first demonstation of such a gas sensor system using a spectraphone (a module for detecting laser-induced sound) consisting of a quartz tuning fork (QTF)/microresonator assembly and two commercial single frequency diode lasers operating at λ =1.58 μm for detecting H 2 S as well as CO 2 and at 1.65 μm for detecting CH 4. A minimum detectable H 2 S concentration of 10 ppmv (parts per million by volume) at the 1σ level was obtained for t=1 s averaging time, scaling down as 1/t ½ up to 1000 s (verified) This implies that a 100 s sampling time will result in a minimum detectable concentration of 1 ppm. For CO 2 and CH 4 , the minimum detectable concentrations were 270 and 1.5 ppm, respectively. The measurement technique will be described and test results will be presented along with implications for the field of riser condition monitoring. This system will allow continuous monitoring of the annulus chemical environment in flexible risers. Current gas monitoring solutions are either offline with a low sampling frequency or require cumbersome EX protection near the pipe (gas chromatography). By contrast, the proposed technique will allow a compact sensing unit connected only with fiberoptics that can monitor annulus gas levels continuously and with high accuracy. Laboratory test results show a high level of measurement accuracy even over short timescales. These results show a clear advantage over conventional systems due to the real-time sampling, and the compact final design being free of electrical leads allows for a compact bolt-on solution which can be installed almost anywhere without compromising working space. This also minimises the number of work-hours necessary near the pipe to maintain the monitoring system. Wells may change fluid composition in a way which brings the problem of corrosion fatigue to a riser never intended for these conditions. For example, wells which initially were sweet may become sour over time. Having a gas monitoring solution in place will allow for real-time risk assessment as well as a warning system for changes in riser annulus conditions. For sour service pipes, a gas monitoring solution will allow up-to-date and highly precise corrosion fatigue calculations. Combining the input from gas monitoring with the data from other sources, such as strain and temperature monitoring, will give unprecedented insight into the field service conditions of a riser, yielding a far greater level of operating safety than previously feasible.
This paper describes the currently available optical condition monitoring options for flexible pipes used in the offshore industry. Temperature monitoring systems for the entire length of the pipe are widely commercially available, while pointbased strain monitoring is a newer technology that is seeing increased field use. However, several upcoming fiberoptic technologies offer new possibilities within riser monitoring. Together with the existing sensing technologies, this will allow unprecedented insight into pipe operating conditions, which in turn will lead to more accurate models for operating conditions as well as far better estimates of the remaining pipe lifetime. Additionally, increased real-time monitoring can become a valuable tool for fault detection and predictive maintenance.
The scope of this paper is to present the results from full scale testing of a flexible riser equipped with embedded sensors for distributed temperature sensing. Testing includes monitoring of accessories mounted on the riser, monitoring of hot spots such as trenched sections of the riser, and finally detection of breaches of the outer sheath. Furthermore, the applications of such a condition monitoring system within Riser Integrity Management (RIM) and production optimization will be discussed. For example, the temperature may be measured along a riser (or flowline) for evaluating the annulus environment and as a supplement to the flow assurance assessment, e.g. to avoid hydrate formation. MotivationWith the increasing use of flexible pipe technology -driven by the move of new offshore fields to deeper, more marginal and more challenging conditions -the need for systematic management of the flexible pipes is becoming more apparent. In particular, the full implementation of Riser Integrity Management (RIM) plays a vital role to ensure an efficient and safe operation. Riser condition monitoring and inspection form an important part of integrity management together with processing and analysis of the monitored data. In that respect, the recently developed technology within optical fiber monitoring offers unique possibilities for quantifying the integrity of the flexible risers during their service life. The advantage of an embedded monitoring system is the ability to continuously survey the riser condition, thus enabling operators to detect potential damages in due time. The embedded system will allow monitoring in critical areas (e.g. under a bend stiffener or inside an I-tube) where access is limited for externally mounted monitoring systems.
Tensile armor wire breaks in flexible risers have been studied by measuring strain in wires during a series of full-scale dynamic pipe tension tests. This has resulted in an understanding of wire breaks described by parameters such as time evolution, spatial distribution and the scale of the measured strain change. With the measured strain data wire break events in flexible risers can be detected and discriminated from other events in the pipe structure. Hence, by instrumenting risers with a grid of fiber optical strain sensors embedded in the tensile wires a Wire Break Detection system can be implemented. The test data and the resulting knowledge of how strain variations from a wire break event propagate in a flexible riser are presented.
This paper presents a new advancement within the field of optical gas measurement with applications to the monitoring of gases inside the annulus of flexible risers used in the offshore industry. This advancement is based on the novel Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) technology. Specifically, we report on the first demonstation of such a gas sensor system using a spectraphone (a module for detecting laser-induced sound) consisting of a quartz tuning fork (QTF)/microresonator assembly and two commercial single frequency diode lasers operating at λ =1.58 μm for detecting H 2 S as well as CO 2 and at 1.65 μm for detecting CH 4 . A minimum detectable H 2 S concentration of 10 ppmv (parts per million by volume) at the 1σ level was obtained for t=1 s averaging time, scaling down as 1/t ½ up to 1000 s (verified) This implies that a 100 s sampling time will result in a minimum detectable concentration of 1 ppm. For CO 2 and CH 4 , the minimum detectable concentrations were 270 and 1.5 ppm, respectively. The measurement technique will be described and test results will be presented along with implications for the field of riser condition monitoring.This system will allow continuous monitoring of the annulus chemical environment in flexible risers. Current gas monitoring solutions are either offline with a low sampling frequency or require cumbersome EX protection near the pipe (gas chromatography). By contrast, the proposed technique will allow a compact sensing unit connected only with fiberoptics that can monitor annulus gas levels continuously and with high accuracy.Laboratory test results show a high level of measurement accuracy even over short timescales. These results show a clear advantage over conventional systems due to the real-time sampling, and the compact final design being free of electrical leads allows for a compact bolt-on solution which can be installed almost anywhere without compromising working space. This also minimises the number of work-hours necessary near the pipe to maintain the monitoring system. Wells may change fluid composition in a way which brings the problem of corrosion fatigue to a riser never intended for these conditions. For example, wells which initially were sweet may become sour over time. Having a gas monitoring solution in place will allow for real-time risk assessment as well as a warning system for changes in riser annulus conditions. For sour service pipes, a gas monitoring solution will allow up-to-date and highly precise corrosion fatigue calculations. Combining the input from gas monitoring with the data from other sources, such as strain and temperature monitoring, will give unprecedented insight into the field service conditions of a riser, yielding a far greater level of operating safety than previously feasible.
Dynamic flexible risers are unique in the sense that they can accommodate substantial curvature variations combined with high tensile loads without compromising their fatigue capacity. However, as the fatigue endurance assessment becomes increasingly challenging with increasing water depth it is of utmost importance to know the fatigue driving mechanisms in flexible risers used for deepwater applications. Thus, key fatigue drivers are discussed in this paper including high inter-layer contact pressure due to severe tension and ambient hydrostatic pressure loads, corrosion fatigue in connection with high strength steel armour, riser structural properties (e.g. stiffness and damping), impact from riser VIV and second order springing effects of the floating unit. Furthermore, helpful fatigue analyses procedures are presented including a methodology based upon transfer functions that can determine the fatigue damage along the length of a riser thus facilitating an efficient irregular wave fatigue assessment. At present, dynamic flexible risers are used in connection with oil and gas floating production facilities in water depth down to approximately 2000m. However, research is ongoing to further expand the flexible riser capabilities thus meeting the requirements of the future offshore field developments at even more demanding water depth between 2000m and 3000m. Thus, it is demonstrated in the paper how a new flexible pipe concept, Flextreme®, having excellent fatigue performance can be used for deep- and ultra-deepwater applications, and how failure modes normally associated with the conventional flexible pipe structure have been eliminated. Finally, ways to manage the risk of fatigue failure during operation of a deepwater flexible riser are addressed, including presentation of a condition monitoring technique using optical fiber technology. Also, a concept based upon active flushing of the riser annulus is described which can be used to extend the service life of in particular sour service risers. Introduction Fatigue assessment of flexible riser systems in general is a challenging design task, involving dynamic system configurtion analyses combined with static pipe cross-sectional stress calculations. Despite the fact that flexible riser systems have been analyzed for a couple of decades substantial resources are still being spent to develop theoretical analysis models that can better describe their fatigue behaviour. As a result, many of the governing fatigue input parameters can now be established with a high degree of accuracy (e.g. design S-N curves, stress distribution of pipe cross-section), thus increasing the confidence in the results. Furthermore, the recent advances in condition monitoring (including optical fiber technology) make it possible to verify the riser integrity during service. However, the originally established safety factor of 10 is often still used as acceptance criterion when assessing the accumulated fatigue damage. This circumstance becomes increasingly challenging with increasing water depth as deepwater riser systems may have difficulties in meeting similar safety factor, in particular when analyzing risers subjected to corrosion fatigue. Thus, it is important to understand the key fatigue driving mechanisms for deepwater riser systems to establish a sound basis for assessing the fatigue endurance with confidence. Thereby, it becomes viable to establish an updated safety factor without compromising the intended reliability of the riser system. Recent work on establishing reliability-based fatigue safety factors can be found in [1] and [2].
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